It is known how to measure the dc resistance of ground, at a grounding rod, since the dc resistance of the earth may change from one season to another, or be affected by moisture or other factors. A grounding rod may extend 20 feet into the earth and have a stub that extends upward above the earth's surface. It is also known how to measure the dc resistance of a grounding current path from a point that is remote from the grounding rod, such as on the third floor of a facility containing electrical or electronic equipment. Both of the foregoing measurements seek to measure the dc resistance of the ground current path. The current path to the grounding rod may extend through a metal strap, steel column, pipe, wire cable or other electrical conductor. At points several stories above the earth the steel columns or pipes may include several sections that have been joined by welds, in the case of columns, or couplers, in the case of pipes.
One known technique, known as the three point measurement method, for measuring dc ground resistance in the earth, at a particular grounding rod, measures the dc resistance of a ground path using a high impedance ohmmeter and two auxiliary grounding rods having about the same dimensions as the grounding rod being measured. The three grounding rods are embedded in the earth, spaced a distance apart of 6 meters or more to avoid overlapping grounding zones. The resistance between each pair of grounding rods is measured with the ohmmeter. The resistance of the desired ground path is then calculated based on the three impedance measurements and a set of well-known equations. Typically, each impedance measurement is made twice, once with the ohmmeter leads reversed, and averaged to avoid electrochemical voltaic effects or other potential measurement inaccuracies.
A second known technique for measuring dc ground resistance is referred to as the voltage-drop or fall of potential method. In this method, two auxiliary grounding rods, a current source, and a voltmeter are used. The first auxiliary grounding rod is spaced a selected distance from the grounding rod to be tested that is large compared to the size of the grounding rod. The second auxiliary grounding rod is located between the first auxiliary ground rod and the grounding rod to be tested. A direct current i is injected into the earth through the first auxiliary grounding rod and a voltage v is then measured between the second auxiliary grounding rod and the grounding rod to be tested. The resistance of the ground path to be tested is then obtained from the equation R=v/i.
One known commercial device for measuring dc ground resistance is available from James G. Biddle Co. under the trade name MEGGER. It uses an ohmmeter, a calibrated voltage source, and one auxiliary grounding rod in addition to the ground path being measured. The resistance is read directly from the ohmmeter.
One problem with these known methods is that they measure only the dc resistance and do not measure or determine ground path impedance, having both resistive and reactive components, at frequencies above dc.
It is known that a ground path to earth for grounding a structure or device has both an ac impedance and a dc resistance. See, e.g. Mardiguian, Grounding and Bonding, Chapter 2 pps. 2.1 et seq., published by Interference Control Technologies, Inc., which states, among other things, that the influence of the reactive component will increase at increasing frequencies and that there may be some frequencies where the ground path circuit becomes resonant.
Mardiguian, Grounding and Bonding, pps. 10.27 to 10.29, and Plumey et al., "High Frequency Harmonic Input Impedance Of An Antenna Embedded In A Conducting Half-Space", Electromagnetic Compatibility, 1983 Proceedings, pp. 45-50, discuss grounding lightning transients and refer to measuring ground impedance at frequencies above dc. The measurement is made using a grounding rod under test, an auxiliary grounding rod providing a current return, a reference grounding rod, a surge generator for injecting a current impulse through a matched resistor between the grounding rod under test and the first auxiliary grounding rod, and an oscilloscope or spectrum analyzer for measuring the voltage transients across a voltage divider network connecting the ground rod to be tested and the second auxiliary grounding rod. The impedance is determined from the ratio v/i of the probe to earth voltage to the injected current transient.
It is known that the impedance of a grounding system is controlled by at least five major factors, skin effect, resonance, antenna effect, bonding, and earth impedance. Skin effect is the tendency of the current to flow in the outer conductive portion of a conductor at higher frequencies. This phenomenon causes the real and imaginary components of the impedance of a ground conductor to increase as the frequency increases. Skin effect in good conductors begins to rise rapidly with frequencies above 100 KHz. Resonances are caused by distributed capacitance in the conductor, proximate metal structures, and distributed inductance caused by bonded joints along the conductor path, self-inductance of grounding wires, as well as deposits in the earth. Antenna effects are related to resonances, and concern the efficiency of the ground path to act as an antenna and radiate or couple to external electromagnetic fields in the atmosphere. Bonding is the quality of bonds between portions of the ground path. For example, a bond having a low dc resistance may have a high impedance at high frequencies, which may result in stray inductive or capacitance reactance and adverse standing wave effects on the ground path. Earth impedance concerns the impedance of the earth in the vicinity of the grounding rod, which can vary with the geographical and environmental conditions, such as soil resistivity, water content and mineral deposits.
One problem with the known methods and devices for measuring ground path resistance is that they are not useful for measuring the ground path impedance at frequencies above dc. Indeed, there are no known commercial devices suitable for conducting such measurements in a practical manner. Further, no standards for conducting such ground impedance measurements at frequencies above dc are known to exist.